The information provided below is not admitted to be prior art to the present invention, but is provided solely to assist the understanding of the reader.
Acrylate, methacrylate and other unsaturated monomers are widely used in coatings, adhesives, sealants, and elastomers, and may be crosslinked by ultraviolet light in the presence of photoinitiators or peroxide-initiated free radical cure. These photoinitiators and/or peroxides are typically low molecular weight multifunctional compounds that may be volatile or readily absorbed through skin and can cause adverse health effects. Functionalized oligomeric photoinitiators may overcome some of these drawbacks; generally, polymeric photoinitiators are nonvolatile compounds, not readily absorbed through skin. However, multistep syntheses may be required, low functionality may be detrimental to reactivity and final properties, and catalyst or initiator may still be required to effect crosslinking.
The Michael addition of acetoacetate donor compounds to multiacrylate receptor compounds to make crosslinked polymers has been described in the literature. For example, Mozner and Rheinberger reported the Michael addition of acetoacetates to triacrylates and tetracrylates. (16 Macromolecular Rapid Communications 135 (1995)). The products formed were crosslinked gels. In one such reaction, depicted in FIG. 1, Mozner added one mole of trimethylol propane triacrylate (TMPTA) having 3 functional groups to one mole of polyethylene glycol (600 molecular weight) diacetoacetate (PEG600-DAA) having two functional groups. (Each acetoacetate “functional group” reacts twice, thus each mole of diacetoacetate has four reactive equivalents.) The resulting network is considered “gelled”, or cured, despite the presence of unreacted acrylic functional groups. While further reaction can be promoted, this network cannot be made liquid either with heat or solvent because it is effectively crosslinked.
The reaction can be characterized by various ratios to describe the reactants: a mole ratio of TMPTA:PEG 600 DAA=1:1, a functional group ratio of the number of acrylate to acetoacetate functional groups=3:2, and a ratio of reactive equivalents=3:4.
U.S. Pat. Nos. 5,945,489 and 6,025,410, to Moy et al., assigned to Ashland, Inc., the assignee of the present invention, disclose that certain organic soluble liquid uncrosslinked oligomers, made by one step Michael addition of β-dicarbonyl donor compounds (e.g., acetoacetates) to multiacrylates, can be further crosslinked using ultraviolet light without requiring costly photoinitiators. The disclosed oligomers may be described as self-photoinitiating acrylate resins. Moreover, when precise proportions of multiacrylate acceptor compounds to β-dicarbonyl donor compounds are combined in the presence of a basic catalyst, liquid oligomeric compositions result. If proportions below the ranges disclosed in the above-cited patent documents are used, crosslinked gels or solid products are made. In addition, the disclosed liquid oligomer compositions can readily be applied to various substrates using conventional coating techniques such as roll or spray prior to ultraviolet light cure.
The term monomer is herein defined as a molecule or compound, usually containing carbon and of relatively low molecular weight and simple structure, which is capable of conversion to polymers, synthetic resins, or elastomers by combination with other similar and/or dissimilar molecules or compounds.
The term oligomer is herein defined as a polymer molecule consisting of only a few similar and/or dissimilar monomer units.
The term resin is herein defined as an oligomer, which is capable of conversion to high molecular weight polymers by combination with other similar and/or dissimilar molecules or compounds.
The term thermoset is herein defined to be a high molecular weight polymer product of resins that solidifies or sets irreversibly when heated. This property is associated with crosslinking reactions of the molecular constituents induced by heat, radiation, and/or chemical catalysis.
Co-pending application Ser. No. 10/628,373, assigned to the assignee of the present invention, the entire disclosure of which is expressly incorporated by reference and for all purposes, discloses a dual-cure thiolene system comprising a self-initiated acrylate resin crosslinked with a polyfunctional thiol.
The utility of acrylate-based resin systems is limited by their relatively poor adhesion to metal substrates. Adhesion to metals can be enhanced by using cationic-cured aliphatic epoxy compounds.